Everyone who visits Kasanka Nationwide Park in Zambia throughout “bat season” agrees that the night emergence of African straw-coloured fruit bats from their roost website is without doubt one of the wildlife wonders of the world. The bats (Eidolon helvum) arrive at Kasanka yearly round October. The numbers swell quickly till they peak in November. By January they’re gone once more.
As soon as they get better from the shock of the breathtaking spectacle, everybody additionally converges on the identical query – what number of bats are there? So many fly out so quick, it appears not possible to depend them. Previous estimates based mostly on visible counts have ranged from 1 million to 10 million, an indication of how troublesome the duty is.
To crack the issue we clearly wanted a new method. Utilizing an array of small video cameras, we filmed the bats leaving their roost after which developed synthetic intelligence to depend them. This gives a cheap, quick and repeatable method to depend giant numbers of shifting animals.
Our common estimate for the Kasanka colony for 5 days in November 2019 was 857,233 bats. This makes it one of many greatest bat colonies on the earth, and a very powerful in Africa.
The subsequent query is why we wished to depend them.
Why counting is vital
Previous work on this species has proven that the ecosystem companies they supply are unparalleled. They disperse seeds each evening over distances of 75km and extra – thrice additional than the African elephant. Bigger colonies disperse extra seeds and are thus extra beneficial in ecosystems. Sadly, decreases in bat numbers are being noticed in locations. Standardised counts are vital to tell apart between colony shifts resulting from disturbance by individuals and population-level declines that require conservation administration.
Counting the Kasanka colony is vital for one more motive too. The African straw-coloured fruit bat is the one long-distance migrant fruit bat on the continent. We don’t know the main points of those migration routes but, however annually we see them converging in momentary colonies, such because the one in Kasanka, after which shifting on to components unknown. Their time at these stop-over websites appears to be synchronised with peaks in native meals availability, and bigger colonies are higher at matching their timing with the perfect meals availability.
So giant colonies point out a more healthy, extra food-rich panorama, and are additionally key to sustaining the collective behaviour of migration.
The expertise wanted to trace the bats and perceive their migration paths remains to be being developed. Only some people have been studied. The outcomes had been nonetheless putting. The bats flew off to many locations throughout the continent, together with one all the way in which to South Sudan. It appears that evidently bats from a number of different colonies meet at Kasanka throughout a short while of the yr, most likely to make the most of the plentiful fruit within the area.
For our new counting method, we determined to movie the bat emergence in an ordinary means, depend the bats in every video, after which extrapolate a complete quantity.
The important thing was gathering information from all sides of the colony. So we surrounded the bat forest with 9 GoPro cameras, aimed straight upwards. These small “helmet cameras” are extra sometimes used to movie excessive sports activities, however had been additionally appropriate to file the darkish bats flying beneath the pale night sky. Our group of scientists and rangers would race across the park to start out the cameras simply earlier than the bats began flying, take a break to marvel on the bat swarm, then return after darkish to fetch the cameras, whereas dodging the hippos and crocodiles that share the swamp forest with the bats.
Again on the lodge there was simply time to recharge the batteries and obtain the footage to arrange for filming the subsequent day.
We bought about 45 hours of footage over 5 days of filming. However we nonetheless needed to truly depend the bats.
Manually counting bats in these movies was not life like – there have been simply too a lot of them. As a substitute, our group developed a synthetic intelligence (AI) program to recognise the bats in opposition to the night sky, observe them from body to border as they flew throughout the display, and depend them at any time when they crossed the centre line.
The AI takes 1.25 minutes to course of one minute of video. This implies 40 hours of footage takes 50 hours to run on the pc. If a human took two minutes to depend all of the bats in a single video body, it will take over 13 years to finish the job.
We checked the accuracy of the AI methodology by manually counting some brief clips and located it was detecting 95% of the bats.
We then used a little bit of trigonometry to determine what portion of the overall colony was flying previous our cameras and extrapolated a complete colony dimension. The best quantity on sooner or later was 987,114 bats.
We would not have caught the colony at peak dimension throughout our 5 days of counting, so we are able to say there are about one million bats in Kasanka at peak season in November.
There are caves in Texas with extra bats, however they’re much smaller. The Kasanka colony of straw colored fruit bats is the most important (by weight) on the earth by not less than an order of magnitude.
Using low-cost GoPro cameras and the modern automated analyses allowed us to ascertain a straightforward methodology (climate allowing) to depend this and different colonies of animals over successive years. We hope this can permit us to establish adjustments in numbers to tell conservation efforts. These are vital as a result of defending the Kasanka colony helps shield bats from your entire sub-continent.
Agricultural developments are encroaching on Kasanka Nationwide Park and a wind farm is deliberate within the space. This might have unfavorable results on the numbers of bats aggregating right here. Monitoring will likely be essential to disclose and stop these results wherever the species offers ecosystem companies.
Hopefully the bats will proceed to darken the night sky of Kasanka for a few years to return, persevering with their companies as the key gardeners of Africa.